Humidity
Sweat cools as it evaporates. Humid air limits that evaporation.
Chicago · October 11, 2026
Heat and marathon pace
A warmer race puts new demands on the body. The physiology, research and interactive pace model show why conditions matter, and what a model can tell us.
One circulation, two demands
Working muscles need oxygen. As the body heats up, more blood also flows to the skin to release heat. Sustaining movement and cooling the body both depend on the circulation.
Heart and circulation
Blood supplies both working muscles and skin.
Working muscles
Oxygen for movement
Blood carries the oxygen needed to sustain the effort of running.
Skin
Blood flow for cooling
More blood carries heat toward the skin. Sweat removes heat as it evaporates.
Cooling adds to the demands of exercise. This diagram explains the mechanism; it does not show measured blood-flow shares or a personal time penalty.
Built with AI · Jenn Umanzorjennumanzor.com · Chicago heat guide
Temperature and pace
An ideal-conditions goal sets the baseline. Changing the air temperature reveals the site’s illustrative pace adjustment, with other conditions held fixed.
Ideal-conditions goal
6:55/miEstimated pace
3:01:09Estimated finish
+1.2%Modeled time
Green line: illustrative pace. Blue marker: selected temperature. Dew point is fixed at 40°F; wind and sun penalties are excluded. This is a planning heuristic, not a measured personal response or a safe-racing threshold.
Exact values and calculation
| Air | Pace / mile | Finish |
|---|---|---|
| 40°F | 6:50 | 2:59:00 |
| 50°F | 6:50 | 2:59:00 |
| 55°F | 6:52 | 3:00:04 |
| 60°F | 6:55 | 3:01:09 |
| 65°F | 7:01 | 3:03:50 |
| 70°F | 7:07 | 3:06:31 |
| 75°F | 7:18 | 3:11:26 |
| 80°F | 7:29 | 3:16:22 |
The model adds 0.12 percentage points per °F above 50 through 60, then 0.30 through 70, then 0.55 above 70. Finish time is the baseline multiplied by that factor. These coefficients are assumptions, not a regression reproduced from the research.
More than a thermometer
Air temperature is only one part of the exposure. Conditions also change over the hours spent on the course.
Sun
Radiant heat adds to the load, even at the same air temperature.
Wind
Air movement changes heat exchange. The thermometer alone cannot describe it.
What the research can tell us
Warmer conditions, slower performances
Ely and colleagues studied marathon results across wet-bulb globe temperature categories and found progressively slower performances in warmer categories.
WBGT accounts for environmental heat stress. It is not interchangeable with the air-temperature slider above.
Ely et al., 2007 · read the abstractCooling has a circulatory cost
Simmons and colleagues review how increased skin blood flow supports heat loss and adds to cardiovascular demands during exercise in the heat.
The mechanism helps explain the challenge. It does not assign a fixed share of blood to the skin or establish one runner’s exact slowdown.
Simmons et al., 2011 · read the reviewResearch → planning model
The research supports the direction of the effect. The calculator’s coefficients are explicit planning assumptions. Neither study establishes Jenn’s exact Chicago time penalty.